1. Field of the Invention
This invention relates to the field of information storage on media designed for optical reading by means of focused radiation and is more particularly directed to an access device having one track carried by a medium which can be optically recorded or read, and to an optical system comprising an access device of this type.
2. Description of the Prior Art
The general progress achieved in laser techniques and thin films as well as in the technique of position-control systems has made it possible to produce an optically readable information medium which has the shape of a disc approximately thirty centimeters in diameter and on which 10.sup.10 bits of information can be recorded.
A medium of this type can be employed in particular as a data-processing memory but it is necessary in this case to provide a device for fast access to the track, either for writing information at a predetermined point of a previously written smooth track or for reading the written information at any point of the track.
Information or data access devices of currently available types have been designed for discs on which the recorded information contains a video signal (video discs); devices of this type permit access to information in a few seconds and are sufficient for this application but no longer prove adequate for a data-processing application.
In known access devices of the prior art, correct radial positioning of the optical reading head is ensured by mechanical means for effecting either the displacement of the disc or the displacement of the reading head. When the reading head and the disc are correctly positioned with respect to each other, the circular or spiral track on which the information is to be recorded or has already been recorded is followed radially by means of a galvanometer mirror. Said mirror is capable of rotating about an axis which is parallel to the plane of the disc and this latter reflects the beam to the reading lens. The operation which consists in vertical position-control so that the reading light beam remains correctly focused on the disc is carried out by means of an electromagnetic coil which produces a vertical displacement of the reading lens.
The weight of the assembly formed by the optical head and the means for radial and vertical position-control can hardly be less than 300 g; and this entire assembly has to be displaced at the time of pre-positioning of the reading head with respect to the disc for fast access to a groove of the track. By reason of the fairly substantial weight to be displaced, the corresponding access times are therefore of long duration.
The present invention is directed to a device for providing access to information in which the moving system has a considerably lower weight than that of moving systems constructed in accordance with the prior art and consequently permits a considerable reduction in access times.